1: Field of the Invention
The present invention relates to a method for estimating an arrival direction or arrival bearing of a radio wave, and more particularly to a method for estimating an arrival direction of a desired wave.
2: Description of the Prior Art
As a method for measuring an arrival direction of a radio wave, or a method of angle measurement, there has been proposed a method employing an array antenna composed of a plurality of element antennas and corresponding signal processing technology. One of the representative method thereof is such one that signals received by respective element antennas are subjected to calculation using an autocorrelation matrix and a MUSIC (MUltiple SIgnal Classification) algorithm so that the arrival direction of the radio wave is estimated. As for the MUSIC algorithm, there can be introduced an article, R. O. Schmidt, xe2x80x9cMultiple Emitter Location and Signal Parameter Estimation,xe2x80x9d IEEE Trans., vol. AP-34, No. 3, pp. 276-280 (March 1986). Further, Japanese Patent Laid-Open Application No. Hei 8-201498 (JP, 08201498, A) discloses a method in which when carrying out angle measurement using the MUSIC algorithm, measurement error can be minimized even if a signal level is low. Japanese Patent Laid-Open Application No. Hei 8-262121 (JP, 08262121, A) discloses a method in which when carrying out angle measurement using the MUSIC algorithm, a virtual image can be suppressed even if a number of radio wave sources are present. Japanese Patent Laid-Open Application No. Hei 9-257901 (JP, 09257901, A) discloses a method in which a MUSIC algorithm employing a space smoothing processing is utilized for improving an angular resolution. And Japanese Patent Laid-Open Application No. Hei 10-177065 (JP, 10177065, A) discloses a method in which an angle of incidence of an incoming wave can be calculated with a smaller calculation cost by using a signal processing apparatus employing the MUSIC algorithm.
As a calculation algorithm for the angle measurement, in addition to the above-introduced MUSIC algorithms, there has been proposed an ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) algorithm. The ESPRIT algorithm is advantageous in yielding a calculation result with a smaller amount of arithmetic operations than the MUSIC algorithm. As for the ESPRIT algorithm, there can be introduced an article, R. Roy and T. Kailath, xe2x80x9cESPRITxe2x80x94Estimation of Signal Parameters via Rotational Invariance Techniquesxe2x80x9d, IEEE Trans., ASSP-37, 7, pp. 984-995 (1989). Also, Japanese Patent Laid-Open Publication No. Hei 10-206518 (JP, 10206518, A) discloses an example of a goniometer employing the ESPRIT algorithm.
However, the above-introduced methods and apparatus encounter the following drawback. That is, if an arrival direction of a radio wave is estimated by using the MUSIC algorithm or the ESPRIT algorithm under an environment in which not only a desired wave but also a reflected wave or interference wave thereof can be received, the arrival direction of the desired wave and the arrival direction of the interference wave are detected in a mixed manner. For this reason, it sometimes become impossible to identify the arrival direction or arrival bearing of the desired wave.
Therefore, an object of the present invention is to provide a method for estimating an arrival direction of a desired wave in which the arrival direction or arrival bearing of the desired wave can be estimated even under the presence of an interference wave or multiple reflection waves.
The above object can be achieved in such a manner that an identical signal series transmission is repeated twice or more from the transmitting side, the transmitted signal series are received by an array antenna composed of a plurality of element antennas at a receiving side, cross-correlation between a pair of received signal series each corresponding to different repetition time is calculated, and then the arrival direction of the radio wave is estimated by using a cross-correlation matrix thereof.
According to the present invention, the same signal series is repetitively transmitted in a continuous manner from the transmitting side, the transmitted signal is received by the array antenna on the receiving side and cross-correlation between the signal series repetitively received in a continuous manner is calculated, whereby it becomes possible to suppress the interference wave or reflected wave even under an environment in which an interference wave, reflected wave or multiple reflection waves are present. Further, if a MUSIC algorithm or the like is applied to the obtained cross-correlation matrix, it becomes possible to extract exclusively the arrival direction of the desired wave.
Even if the reflected waves arrive at different timings under the environment of presence of multiple reflection waves, when signal reception is carried out while attention is paid to a specific single reflected wave, the correlation of the signal series with the signal series deriving from the same reflected wave becomes large while the correlation of the signal series with another reflected wave becomes very small. Therefore, an arrival direction can be independently identified for each reflected wave by taking an advantage of the difference in arriving timing.